Two basic methods of producing material columns in the ground have been known for a long time. In the “wet tamping compaction” method, a column-like hole is produced and then flushing water is charged into the hole. Rapidly moving a vibrator up and down, in combination with the water flushing results in a flow which erodes the soil material in such a way that an annular space is created between the depth vibrator and the adjoining soil region, thereby making it possible to transport gravel, recycled concrete, sand or similar granular bulk material along the depth vibrator and into the hole. Apart from the eroding action, the water performs further important functions as well, namely that of stabilizing the hole wall against any collapse of the wall caused by the pressure of the groundwater and that of preventing gravel from jamming between the hole wall and the depth vibrator. The upwardly flowing water eliminates these “bridges” formed by jamming and thus ensures that the gravel is transported depthwise down to the lower end of the vibrator, where it is tamped to form a so-called “tamped column”. A particular problem associated with this method is that of disposing of the extremely sludgy flushing water, something which entails high costs inter alia, and another problem is that contamination which may be present in the soil is washed out of the soil to the surface.
In a second method, the “dry tamping compaction” method, a material-conveying pipe is fastened laterally alongside a vibrator and is used to transport material to the lower end of the depth vibrator. Disadvantages in relation to the wet tamping compaction technique are the lower gravel-conveying capacity (volume per time) and the higher costs for the more elaborate equipment.
In both methods, the material is transported to the lower end of the vibrator solely under the force of gravity acting on the material. Without flushing water or without a conveying pipe it is not possible for the gravel to be conveyed into soil layers underneath the ground water level. To accelerate transportation, or if the transportation of material has come to a standstill, the only remaining option is to enlarge the annular space, although this is in most cases undesirable since it results in soil being unnecessarily washed out and in an increased requirement for flushing water.
Accordingly, it would be desirable to provide a device for and a method of producing tamped columns in which, by contrast with the prior art, there is no requirement, or only a relatively low requirement, for flushing water and no conveying pipe is necessary either.